This invention relates to apparatus for converting a continuous laser beam into several pulsed laser beams aimed at different targets. More particularly, this invention relates to an optical system for interrupting a continuous laser beam and dividing it into several pulsed beams directed at a moving paper web for the purpose of perforating multiple rows of holes in the web.
In the manufacture of cigarettes, for example, it is a common practice to perforate the cigarette paper or tipping paper in rows, for the purpose of providing smoke dilution. Before the laser became available, the perforations were made by mechanical means or with electrical spark perforators. With the advent of the laser, it became possible to achieve greater precision and speed than had previously been possible.
In creating a row of perforations using a laser, it is usually the practice to move the paper web while pulsing the laser beam. The maximum speed of the process is thus governed by the frequency at which the beam can be pulsed and the speed at which the web can be moved. With currently available laser equipment, the internal pulsation frequency of the laser is a limiting factor. Because of this, other means, external to the laser, for interrupting a continuous beam have been sought.
In addition, it is frequently desired to perforate the web at more than one location at a time. For example, it may be preferable to have two or more parallel rows of perforations. However, because of the increased capital and operating expenses, and space limitations, it it not desirable to use more than one laser to achieve such perforations.
Many optical systems have been developed to direct one beam to several locations simultaneously. In some systems, the splitting of the beam is accomplished by one or more semi-transmissive mirrors, or "beam splitters." A fraction of the radiation incident on each beam splitter is reflected to form a first beam, a certain small amount is absorbed, and the remainder is transmitted to form a second beam. The reflected beam, or the transmitted beam, or both, may be split again to form as many additional beams as are desired. Each beam so produced is then used to create a perforation in the paper.
The use of semi-transmissive mirrors as beam splitters has a number of serious drawbacks when applied to the task of perforating cigarette paper. The perforations formed in the cigarette paper should be uniformly sized to obtain the proper level of ventilation and other desired characteristics in the cigarette. As is well known to those in the art, however, it is difficult to cause the two beams provided by a beam splitter to have exactly a specific intensity ratio. As a result, the beams so obtained are very likely to produce holes of different sizes.
Other systems use mechanical rotary shutters, or "beam choppers," to periodically interrupt the beam at the desired pulsation frequency. Portions of the shutters may be mirrored in order to direct the pulsed beam to several different targets sequentially. However, when a beam is pulsed with a mechanical shutter, approximately half of the incident beam energy may be wasted during the "off" portions of the cycle. Still another type of beam chopping system diverts the beam to a heat sink, or "beam dump," during the "off" portions of the cycle. In such a system, up to 90% of the beam energy may be wasted.